The earliest organic light-emitting device (OLED) has been proposed, in 1963, by Pope et al, who applies a 1000 V voltage to both ends of an anthracene crystal with a thickness of 1 mm and observed light emission. However, the operation voltage is so high that it is not applicable to a flat panel display. The structure and the manufacturing method of the organic light-emitting device nowadays are proposed by C. W. Tang and S. A. VanSlyke of the Eastman Kodak Company. One may manufacture the structure by sequentially depositing non-crystalline thin films of organic materials, with vacuum deposition on a glass substrate that is pre-coated with a transparent electrode (lower electrode) of Indium Tin Oxide (ITO), and lastly depositing a metal electrode (upper electrode) thereon. The operation voltage of the organic light-emitting device manufactured in accordance with the above method is reduced to within 10 volts, which largely enhances its value in practical use. Also, the vacuum deposition method is suitable for mass production of flat panel displays with large display areas. Furthermore, since the organic light-emitting device has the features of fast response, self-emitting and low process temperature, the OLED has been evolved to play a significant role in the industry of flat panel displays.
Referring to FIG. 1, the energy level structure for each layer of the conventional two layer organic light-emitting device 1 is illustrated. As shown in the figure, the organic light-emitting device 1 comprises: an anode 10, a cathode 14, a hole-transport layer (HTL) 101, and an electron-transport layer (ETL) 109. Holes 12 and electrons 16 are injected into the HTL 101 and the ETL 109, respectively, via the anode 10 and the cathode 14. When the electrons 16 and the holes 12 drift respectively to the junction between the HTL 101 and the ETL 109, the electrons 16 and the holes 12 combine to form excitons and then emit light. However, such organic light-emitting device has a fixed structure. Once it is fabricated, the structure may no longer be changed. Thus this kind of organic light-emitting device cannot emit light of various colors but only can emit light of one single color.
Since the organic light-emitting device is one of the major components of a flat panel display, whether the flat panel display can display in full-color depends heavily on whether the organic light-emitting device is capable of emitting full-colors. However, after the conventional organic light-emitting device is fabricated, its emission spectrum and device characteristics are fixed. To enable the flat panel display to display in full-color, one of the following methods must be employed: (1) fabricating organic light-emitting devices that emit red, green and blue light, respectively, on a substrate so as to form an array of color pixels; (2) fabricating an array of organic light-emitting devices that emit white light or single-color light on one substrate, and incorporating a color control or conversion array previously fabricated on the same or another substrate so as to achieve a full-color display.
In order to achieve a full-color display by the aforementioned method (1), Nagayama et al. have disclosed a precise shadow-mask alignment method in U.S. Pat. No. 5,701,055, No. 5,742,129, No. 5,952,037 and No. 6,373,182 B1. The precise shadow-mask alignment method is used for fabricating organic light-emitting devices of small molecules in high vacuum. In order to fabricate independent organic light-emitting devices that emit red, green and blue light on the same substrate in sequence, one may carry it out by the precise shadow-mask and alignment method in a vacuum or vapor deposition process. The full-color display is achieved by vacuum depositing organic light-emitting devices with different structures that emit different colors in various regions. However, the employment of this method would encounter problems such as shadow-mask alignment errors, vapor deposition shadowing errors during vapor deposition, difficulty in fabricating shadow masks with small openings, insufficient mechanical strength in large-area shadow masks, and the cleaning of the masks. These problems would reduce the resolutions and yields of the display fabricated with this method.
In order to achieve a full-color display by the aforementioned method (1), Wolk et al. have disclosed a method of thermal transfer of organic materials in U.S. Pat. No. 6,114,088. The method of thermal transfer of organic materials comprises fabricating a layer of organic material on a substrate having a light-to-heat conversion layer, and locally increasing the temperature on the light-to-heat conversion layer by shining light thereon, so as to change the adhesion between the interfaces of different materials corresponding to the temperature change, and to induce thermal transfer. The locally heated region of the organic material layer is then transferred from the original substrate to another substrate for display fabrication. In order to fabricate a full-color display, the organic material layers of red, green and blue colors are sequentially transferred to different pixel locations of the substrate. However, the employment of this method would encounter the problems of complicated manufacturing processes and unsatisfactory yield of the fabricated displays.
In order to achieve a full-color display by the aforementioned method (2), one can make only the organic light-emitting device that emits white light for the entire display. The full-color display is achieved by using the color filter array of red, green and blue colors fabricated thereon so as to filter out different colors in various pixel regions of the display (referring to Kido et al, Science 267, 1332 (1995)). However, the intensity of the light obtained through the color filter, in accordance with this method, would lose more than two thirds of that of the white light emitted by the organic light-emitting device. Thus, the light-emitting efficiency would be largely reduced for the display being fabricated with this method.
In order to achieve a full-color display by the aforementioned method (2), Eida et al. have disclosed an apparatus of organic light-emitting device capable of emitting multi-colors in U.S. Pat. No. 5,909,081. For the entire display, one makes only the organic light-emitting device emitting blue/ultraviolet light. By means of a color conversion array made by phosphorescent or fluorescent material that absorbs blue/ultraviolet light, the blue/ultraviolet light may be converted to blue, green or red light. However, the display of the organic light-emitting device fabricated with this method would encounter the problem of energy conversion efficiency due to the light conversion layer.